Happened to find a short essay on "Why Rap is Rock" written by a blogger. Seems to have some merit; if you're so inclined, give it a chance. I've only cut and pasted a few paragraphs, but I strongly encourage you to read the whole thing -- it's comparable to two book pages, max; it's a very quick read. Informal, nothing scientific, very entertaining, and lastly, it makes sense.
Why Rap is Rock
To tell the truth, you can almost understand how people honestly fail to see just how rap fits into the larger rock puzzle, mainly due to the fact they're basing their entire definition of rock on something else from the same time rap appeared rather than tracing rock's evolution in all directions through the years.
You can even comprehend people trying to keep rap out for more nefarious reasons having to do with defending their own preferences from something that threatens to overtake those tastes in impact and popularity. But any way you look at it hip-hop is far more connected to the origins of rock 'n' roll than heavy metal or progressive or alternative ever were for instance, and thus demands to be included equally in the discussion. If you wanna get rid of 'em all in the rock family tree you'd have an easier time defending that decision than if you choose just any one of them individually to cast aside. But why discard any of them when they add so much texture to the ever growing fabric of rock 'n' roll in the first place?
Like any creative entity rock 'n' roll has evolved in ways that nobody could've foreseen when it began in 1948 and as a result it has grown very wide shoulders, which is precisely what has allowed it to remain the most culturally relevant music through the sixty-plus years it's been around.
With each new stylistic shift and every new plateau attained along the way rock's hold on the public's imagination only deepened. In the process of this remarkable evolutionary growth rock has, at times, seemed to become too unwieldy for many people to grasp, and so, selfishly and capriciously, they've continually attempted to narrow its parameters in order to satisfy their own musical preferences and resist the tidal wave crashing into their cultural prejudices. The problem with this method of selective inclusion is that it EXcludes some of rock's most dynamic and vital creations which in turn weakens its very existence and if allowed to continue would eventually destroy the viability of the rock genre entirely.
How is this possible? Hasn't rock's lengthy history proven that it is almost indestructible? Sure, but only because it hasn't shunned its various creative differences as they've sprung up but rather fully embraced them and welcomed them into the fold, ensuring that successive generations will continue to identify with and want to be associated with the term rock music because rock has allowed itself to grow along with them.
Naturally what began as a fairly uncluttered field in the late 40's began to grow exponentially over the course of just a few years so that as more and more artists emerged, borrowing from what immediately preceded them but then adding their own creative experiments, it soon came to resemble nothing like that which had been commonplace only a few short years before, yet was intrinsically tied to it nevertheless. Multiply that trend by dozens of new emerging styles for each short musical life span that take place every few years, then multiply that growth over six decades of evolution and you wind up with what you have today, where rock 'n' roll has branched out so far stylistically that the various offshoots seem utterly alien to one another.
Yet in truth they all stem directly from the same root source and all have the same basic goal in mind, to be a cultural prism for the current musical generation, and as a result, from punk to funk, doo wo-p to hip hop, they are all part of the ever-growing rock 'n' roll family.